Eukaryotes constantly interact with an abundant array of different microbes. These interactions can drastically impact the fate, evolution, and responses of both partners in multifaceted ways. Understanding the diverse interactions between bacteria and eukaryotes and their underlying mechanisms can provide insight into host resistance strategies, bacterial pathogenesis, and mutual cooperation. Interestingly, carrier clones of the social amoeba Dictyostelium discoideum have been shown to harbor a variety of bacteria, in contrast to their non-carrier counterparts. In the host, these bacteria provide food and defense. In non-carrier clones, these bacteria are pathogenic. D. discoideum is amenable to a variety of biological techniques, can be infected by important human pathogens, and displays distinct and diverse interactions with a variety of microbial species. These attributes make it a powerful system for probing the eukaryote-bacteria interface. The purpose of my research is to define the differential interactions between D. discoideum clones with bacterial species, and to identify the genes and co-evolutionary trajectories underlying these interactions and their associated outcomes. My specific aims are to: 1. Determine the differential responses of carrier and non-carrier Dictyostelium discoideum amoeba clones to bacterial pathogenesis. 2. Examine the impact of amoeba- bacteria co-evolution on both partners. 3. Identify genes involved in the amoeba- bacteria interface. Through this work I will set up the framework to better define this model system, gain insight into evolutionary pressures in the development and breakdown of eukaryote-microbe cooperation, and identify genes involved in host-microbe mutualism. This flexible system can be further applied to refine our understanding of the evolutionary pathways that specify the fine line between infection and colonization/cooperation and the role of differential host-responses in these events. Finally, this work will pave the way for more in-depth analysis of the molecular mechanisms involved in the amoeba-bacterial interface, thereby allowing for greater resolution of the dynamic causes and consequences of trans- kingdom interactions along the wider symbiosis continuum.